Spinels are solid materials of varying composition in which the unit cell is a closely packed cubic arrangement of 32 anions with 24 cations distributed in interstices between the anions. These materials may be represented by the general formula: EQU X.sub.2 Y.sub.n O.sub.2n+2.
When n equals 1, the material is said to be stoichiometric. Real material, however, may deviate to some extent from the stoichiometric composition. In inverse spinels, the ions X are divalent and the ions Y are tetravalent. In these inverse spinels, one-half of the divalent ions enter the structure in a tetrahedral configuration, while the other half of the divalent ions together with all of the tetravalent ions are present in the structure in octahedral coordination.
The materials which may be considered for use in developing a spinel are those for which the ionic radius is greater than 0.6 angstroms but less than 1.0 angstroms. Thus, ions such as iron, zinc, magnesium, nickel, copper, cobalt, manganese, titanium, tin, silicon, vanadium may be considered.
Blue and green pigments of this system have been known for some time. For example, combinations of nickel oxide, cobalt oxide and zinc oxide together with titanium dioxide have been sold as a light green pigment. In U.S. Pat. No. 3,424,551 are disclosed blue and green pigments made from mixtures of titanium, magnesium, cobalt and zinc oxides. In addition, certain specific compounds such as Zn.sub.2 TiO.sub.4, Mg.sub.2 TiO.sub.4 and Co.sub.2 TiO.sub.4 are discussed in some detail in the scientific literature (for example, see the book, "The Major Ternary Structural Families" by O. Muller and R. Roy, particularly pages 254 and 255).
Quite surprisingly, however, I have discovered that within certain specified composition ranges, it is possible to produce orange pigments within the general framework of the inverse spinel system.